1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the invention relates to an improvement of a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer.
2. Description of Prior Arts
A magnetic recording medium such as an audio-tape, a video-tape, or a recording medium employed in a computer system, basically comprises a nonmagnetic support and a magnetic recording layer provided on the support. The magnetic recording layer comprises a ferromagnetic powder such as a needle crystalline powder of .gamma.-Fe.sub.2 O.sub.3, Co-containing ferromagnetic iron oxide, or CrO.sub.2 dispersed in a binder. Recently, the demand for a higher density recording system has increased, and hence studies of a magnetic recording medium using a ferromagnetic metal powder containing metallic iron, nickel, cobalt or the like, in place of the conventional oxide-type ferromagnetic powder have been made.
The ferromagnetic metal powder has been employed as a ferromagnetic powder of a recording medium of a computer system. The ferromagnetic metal powder is high in a coercive force (Hc) and a residual flux density (Br) and moreover is stable in a wide temperature range. For these reasons, the ferromagnetic metal powder has been also paid attention to for the use in such magnetic recording media as a video-tape or an audio-tape.
Particularly in the case of a video-tape, recently the recording wavelength has been shortened, and further the width of track has been narrowed. Accordingly, the demand for higher density recording is particularly high in the video-tape art.
It is known that in the magnetic recording layer using a ferromagnetic metal powder, the higher density recording and improved electromagnetic conversion characteristics are attained by smoothing the surface of the magnetic recording layer. It is also known, however, that the smoothened surface of the recording layer results in an increase of friction between the surface and the members of a recording device against which the recording medium runs. The increase of friction causes damage to the magnetic recording layer and further may cause dropping off of the recording layer from the support. Particularly, a video-tape sometimes is placed under severe conditions such as the so-called "still mode", and the increase of the friction shortens endurance of the recording layer in the still mode (i.e., still life). Accordingly, improvement of the running endurance (or running property), namely, endurance in the running operation, is desired.
It is known that the running endurance of a magnetic recording layer is improved by incorporating an abrasive (i.e., hard particles) such as corundum, silicon carbide or chromium oxide in the recording layer. However, the effect of incorporation of an abrasive is shown only where the abrasive is incorporated in a large amount, and a magnetic recording layer containing a large amount of an abrasive likely causes extreme abrasion of a magnetic head placed in contact with the running recording layer. Further, the incorporation of a large amount of an abrasive is unfavorable to the electromagnetic conversion characteristics of the recording medium. Furthermore, the incorporated abrasive is apt to be easily released to adhere to the surface of the magnetic head.
Moreover, there are other problems in the art of a video-tape. Recently, the potable video tape recorder system has become popular, and is employed under various conditions such as at a very low temperature or under a very humid condition. Therefore, it is desired that a magnetic recording medium such as video tape functions satisfactorily even under the severe conditions. Thus, a magnetic recording medium is desired to show not only high electromagnetic conversion characteristics and high endurance under severe running conditions such as the still mode, but also high running endurance under wide variation of the surrounding conditions such as wide variation of ambient temperature and humidity.